Investigation of systemic arthritis in populations To investigate whether genetic variation influences sJIA susceptibility on a population level, we partnered with the International Childhood Arthritis Genetics Consortium (INCHARGE) to assemble a collection of 988 children with sJIA and 8010 control subjects from North America, South America and Europe. Using this cohort, we generated a dataset of over 6.7 million common genetic markers or single nucleotide polymorphisms (SNPs) and performed a genome-wide association study (GWAS) of sJIA. We divided the collection into 9 groups by country of origin and examined genetic associations with sJIA separately in each group. Using association meta-analysis, we identified genetic variants whose influence on sJIA risk transcended ethnicity. In previous reporting periods, we established that variants of the major histocompatibility complex (MHC) locus are the strongest genetic risk factors for sJIA. We showed that SNPs in the class II human leukocyte antigen (HLA) region of the MHC lead to increased risk of developing sJIA with an odds ratio of 2.6. We further showed that these SNPs are in strong linkage disequilibrium with HLA-DRB1*11 alleles and reside on a class II HLA haplotype of HLA-DRB1*11/HLA-DQA1*05/HLA-DQB1*03. Beyond the HLA locus, we identified 21 novel genetic regions that contribute to sJIA susceptibility (p < 1 times 10 to the negative 5). The strongest of these susceptibility loci is an intergenic region of chromosome 1, where SNPs lead to increased risk of sJIA with an odds ratio of 2.4. Observing that none of the new sJIA risk loci intersected with known JIA susceptibility loci, we performed formal genetic comparisons of sJIA with other forms of JIA. These analyses demonstrated that sJIA is genetically distinct from the other forms of JIA. Based on its unique phenotype and distinct genetic architecture, we advocate for the removal of systemic arthritis from the JIA classification scheme. In the current reporting period, we have leveraged the INCHARGE GWAS to investigate 11 sJIA candidate susceptibility loci that were reported in the pre-GWAS era. These loci were identified by small case-control studies of candidate SNPs that were susceptible to sampling bias and population stratification. Moreover, most of these associations were not replicated in subsequent independent studies. Despite these facts, these loci are often centrally featured in discussions of sJIA pathophysiology, which inspired this study. In the INCHARGE collection, we found no evidence of genetic association at 10 of the 11 susceptibility loci. The only association signal observed in our study was at the interleukin-1 receptor antagonist gene (IL1RN) locus, which was significantly associated with sJIA. The disease-associated variants are known regulatory variants that correlate with IL1RN gene expression and serum levels of the interleukin-1 receptor antagonist protein (IL1Ra). Using in silico analyses of published gene expression studies in healthy subjects, we found that the sJIA associated variants were among the strongest predictors of IL1RN expression, with low expression correlating with increased risk of sJIA. Knowing that recombinant form of the IL1Ra protein (anakinra) is an effective treatment for sJIA in some patients, we hypothesized that these genetic markers may predict response to anakinra treatment in sJIA. We observed that in a group of 38 children from the INCHARGE collection who received anakinra treatment, homozygosity for the high expression IL1RN genotypes in sJIA patients was highly predictive of non-response to anakinra treatment (p = 7.7 times 10 to the negative 4; odds ratio 28.7, 95% confidence interval 3.2 to 256; sensitivity 0.92; specificity 0.71). Based on this finding, these SNPs may be clinically useful as biomarkers to guide the personalized treatment of systemic arthritis. We published these findings in Arthritis & Rheumatology. Looking forward, we will continue to use population-based genomic approaches to gain insights into the pathogenesis and pathophysiology of sJIA. To this end, we have partnered with the Juvenile Arthritis Consortium for Immunochip (JACI) to perform an Immunochip-based association study of sJIA. The Immunochip is a commercially available SNP array designed to intensively interrogate genetic variation at over 200 immunologically important loci. We have generated an Immunochip dataset that includes 889 sJIA cases and 16,114 healthy subjects. Preliminary analyses of this dataset have identified a novel sJIA susceptibility locus. We are currently performing follow-up functional studies of this locus to determine the mechanism through which these variants influence disease risk. Investigation of systemic arthritis in individuals To investigate whether genetic variation influences sJIA susceptibility on an individual level, we are using sequencing-based approaches to interrogate rare genetic variation in subjects with sJIA. We are leveraging a subset of the INCHARGE collection to perform a targeted deep resequencing study to evaluate whether rare variants of sJIA candidate genes influence disease risk. The candidate genes are divided into 3 groups: genes that showed at least a modest association with sJIA in our GWAS; genes known to cause monogenic autoinflammatory syndromes, which are phenotypically similar to sJIA; and genes known to cause familial forms of hemophagocytic lymphohistiocytosis, a condition that frequently develops in sJIA patients. Our analyses of this dataset have produced intriguing preliminary results and we are actively undertaking follow-up investigations to understand their implications. Beyond the INCHARGE collection, we have continued to expand our investigations of individuals and families with systemic arthritis. In April 2018, we launched an NIH Intramural Research Program research protocol entitled Investigation of the Natural History, Genetics, and Pathophysiology of Systemic Juvenile Idiopathic Arthritis, Adult-Onset Stills Disease and Related Inflammatory Conditions (18-AR-0081). This protocol is designed to facilitate broad recruitment of children, adults and families with systemic arthritis to the NIH Clinical Center for family-based genomic sequencing, coupled with systematic, longitudinal characterization of clinical and immunological phenotypes.